1. Technical Field of the Invention
This invention relates to a drill bit structure having an improved cutting end construction and particularly it relates to a drill bit structure having a hard tip fitted and fixed in a groove formed in the cutting end of the drill bit body.
2. Description of the Prior Art
Generally, drill bits of high speed steel have heretofore been used in drilling steel and cast iron work pieces. However, today when there is much need to maximize the efficiency of drilling operations, there are many cases where the drill rpm (cutting speed) is increased to meet this need. Under such circumstances, increasing use is being made, as a drill bit material, of cemented carbide, which is superior in wear resistance. However, cemented carbide has an inferior cross-break strength as compared to high speed steel and hence it is not a satisfactory material so far as the strength required for withstanding the cutting resistance is concerned. For these reasons, with the same construction as in conventional high speed drills it is impossible to fully develop the performance of cemented carbide drill bits and they can be used only under moderate cutting conditions.
In drilling operations, the quality of the chip ejecting function or ability influences the cutting resistance. The greater the drilling depth, the greater the cutting resistance, thus making it necessary to improve the chip ejecting ability so as to prevent an increase in the cutting resistance. This is a matter of great importance particularly to cemented carbide drill bits.
A drill bit which has solved the aforesaid problems of the cross-break strength and of the chip ejecting ability is disclosed in European Patent Publication No. 0,127,009, wherein the entire drill bit is made of cemented carbide alloy or the like. European Patent Publication No. 0,127,009 corresponds to the above U.S. Pat. No. 4,583,888.
Thus, if the drill bit diameter is comparatively small and hence the bulk of the entire drill bit is not much, making the entire drill bit of a hard material, e.g., a cemented carbide alloy, would not cost much since the amount of cemented carbide alloy used is small and so is the number of processing steps involved.
However, where medium- and large-sized drill bits are to be produced, making such drill bits entirely of a cemented carbide alloy would naturally result in an increased amount of cemented carbide alloy used and in an increased number of processing steps involved, leading to the disadvantage of high costs.
A drill bit construction entirely different from that of the aforesaid drill bit is disclosed in Japanese Patent Publication No. 116795/1979, wherein a throwaway tip is attached to the cutting end of the drill bit structure. However, this type of conventional throwaway tip is so constructed that a plurality of throwaway tips, rather than a single one, form the cutting lips. If, therefore, the drilling speed is increased to perform high speed drilling, there would be a problem when the cutting end of the drill bit body chatters. Further, where the tips are not symmetrically attached to the drill bit body, there would be a problem in assuring a balance of symmetry with respect to the center of rotation of the drill bit structure. Further, because of the low attaching strength, there has been a problem in the form of a severe vibration produced during drilling.
An example of a drill bit having a cemented carbide alloy tip attached to the cutting end is disclosed in Japanese Patent Publication No. 46928/1981, wherein the cutting end of the drill bit body has a groove and a plate-like tip of cemented carbide alloy is fitted and fixed in said groove, whereby a single cemented carbide alloy tip forms the lips in their entirety and hence the vibration disadvantage found in the drill bit of Japanese Patent Publication No. 116795/1979 described above, has been eliminated.
In the drill bits disclosed in both Japanese Patent Publications Nos. 46928/1981 and 116795/1979, however, have not solved the aforesaid problems of the cross-break strength and of a poor chip ejecting ability.